In a gas turbine engine power generating machine, air is initially compressed by an air compressor, is subsequently heated in a combustion chamber, and the combustion gas so produced passes to a turbine that, driven by the combustion gas, does work which may include rotating the air compressor. Generally, the combustion gas flows out of the turbine and into a diffuser, which expands the flow of combustion gas in a diffusing flow path before exiting the gas turbine engine or entering a subsequent component, such as a Heat Recovery Steam Generator, for example.
In conventional gas turbine engine diffusers, the combustion gas flowing through the diffusing flow path typically includes a boundary layer usually comprised of stagnant fluid which is a “separated” region along an outer surface of the diffusing flow path, a low velocity boundary layer fluid adjacent to the stagnant fluid, and a mainstream fluid adjacent to the boundary layer in a central region of the flow path. As the combustion gas passes through the diffuser, the velocity of the mainstream fluid is reduced as the cross-sectional area of the diffusing flow path expands. However, based on the reduction in velocity of the mainstream fluid, the velocity of the boundary layer fluid is also reduced, the region of the stagnant fluid within the boundary layer tends to increase and the boundary layer becomes more separated from the surface of the diffuser, thereby reducing the effective cross-sectional area available for diffusion of mainstream fluid within the diffuser. A number of conventional approaches have been offered to control the separation of the boundary layer within the diffusing flow path.
One conventional approach involves positioning flow obstruction devices, such as triangular-shaped delta wings or prism-shaped wedges, within the mainstream flow of the diffusing flow path, to deflect the mainstream flow into the boundary layer, in an effort to energize the boundary layer. However, the effectiveness of the flow obstruction devices is dictated by the boundary layer thickness of the boundary layer at the flow obstruction devices and the orientation of the flow obstruction devices relative to the flow angle of the mainstream fluid. Additionally, since the flow obstruction devices utilize the momentum of the mainstream fluid, they introduce an undesirable total-pressure loss on the mainstream fluid.
Another conventional approach involves a blower arrangement in which an opening in a surface of the diffuser is used to introduce fluid against the boundary layer. This fluid impacts the stagnant fluid within the boundary layer but does not, however, have the necessary characteristics to mix the mainstream fluid adjacent to the boundary layer with the stagnant fluid in the boundary layer.
Thus, it would be advantageous to control the stagnant fluid in the boundary layer within the diffusing flow path and improve the effectiveness of the diffuser, without the necessary drawbacks of the conventional approaches articulated above.